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REVIEW - Rigol DS2072 - First Impressions of the DS2000 series from Rigol
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Galaxyrise:

--- Quote from: marmad on November 17, 2013, 04:17:35 am ---So yes, just like other manufacturers' implementations of High Resolution mode, Rigol's implementation acts exactly the same way - and will filter the waveform (and cause anti-aliasing if the effective sample rate is reduced too far for the incoming signal).

--- End quote ---

Compare these two:

and


See that the sample rate is 1Gsa/s in my screenshot, which should leave plenty of effective sample rate for a 100kHz signal.  Agilent produces the results I expected, and with far less sample rate.  As far as I can see, the Rigol is not acting the same way as the Agilent.

The complaint isn't just that "high res" happens post acquisition, but that Rigol's "one pixel per column" algorithm aliases out components that are too high frequency for the current time base.
Tasman:

--- Quote from: Galaxyrise on November 17, 2013, 07:50:13 am ---The complaint isn't just that "high res" happens post acquisition, but that Rigol's "one pixel per column" algorithm aliases out components that are too high frequency for the current time base.

--- End quote ---

What's the point of using high resolution mode for display of an envelope?  If you use normal aquisition mode the Rigol display looks exactly the same as Agilent and accomplishes all you could wish for. 
marmad:

--- Quote from: Galaxyrise on November 17, 2013, 07:50:13 am ---See that the sample rate is 1Gsa/s in my screenshot, which should leave plenty of effective sample rate for a 100kHz signal.
--- End quote ---

Using High Resolution mode applies an LPF function with a -3dB point approximated by 0.433 * sample rate / number of samples. To get 12 bits resolution, it means 4^4, or 256 samples need to be averaged, so that means the best-case (12-bit) bandwidth for either the Agilent or the Rigol when sampling at 1GSa/s is 1.69MHz (or 3.38MHz @ 2GSa/s).


--- Quote ---Agilent produces the results I expected, and with far less sample rate.
--- End quote ---

The Agilent is displaying the number '25MSa/s' because that's the 'normal' sample rate of the DSO at the 1ms time base, but that's not what it's actually sampling at when you're in High Res mode.


--- Quote ---As far as I can see, the Rigol is not acting the same way as the Agilent.
--- End quote ---

True, but Agilent has it's (HP-patented) anti-aliasing which is on automatically at slower time base settings, so it's hard to know exactly if it's random decimation is affecting what is displayed in that image.

It seems like the Rigol's BW in High Res mode is about equivalent to the 3.38MHz mentioned above at the 5us time base setting - then approximately reducing by a factor of 2 for each subsequent lower time base setting.
Galaxyrise:

--- Quote from: Tasman on November 17, 2013, 09:49:58 am ---What's the point of using high resolution mode for display of an envelope?  If you use normal aquisition mode the Rigol display looks exactly the same as Agilent and accomplishes all you could wish for.

--- End quote ---
That example was to demonstrate the difference between algorithms; to demonstrate that Rigol's display with High Res enabled can be very different than other scopes.  If someone expects high res to act like normal but with a lower sample rate and "high res" samples, then they can be surprised on the Rigol. 

Naturally, now that I know more what it does, it can be a useful tool.  One just has to be aware that at 1ms/div + high res, the effective bandwidth (on screen) is like 5kHz.  High res is something I typically use after I've seen what it looks like in normal, and it can be great. I also like combining high res and roll mode.
marmad:

--- Quote from: evanh on November 17, 2013, 07:06:03 am ---"within the same acquisition" means oversampling.  Oversampling is done at acquisition time before storing each high-res sample.
--- End quote ---

1) Neither the Rigol DS2000 nor the Agilent X-Series can sample faster than 2GSa/s, so oversampling is meaningless if the rate is already at (or close to) this speed.

2) There is no theoretical difference between doing the averaging between acquisition and sample memory - or between sample and display memory (although it certainly can have an effect on the speed of the whole process).

3) The big problem with the Rigol is that their documentation is not clear on the subject.

It's impossible to change the memory depth on the Agilent, so at slower speeds, the number of bits of higher resolution is fixed. For example, at 5us/div it's 10 bits. But the Rigol allows user-definable memory depths, so it's obvious that it has to alter the number of bits of resolution at a given time base depending on the memory depth.

For example, according to the Rigol manual, the DSO gives "12 bits of resolution when >=5us/div @ 1GSa/s", but what happens when the sample depth is set to 14k @ 5us/div (meaning the displayed sample rate is 200MSa/s)?

It's clear the Rigol must switch to 10 (or 9) bits of resolution, and this can be observed by testing the LPF of the High Res mode while sending a 4MHz signal into the scope with those settings: 12 bits of resolution while sampling @ 200MSa/s would mean an effective bandwidth of ~338kHz.

So you MUST keep the memory depth set to 14MB (or AUTO) to get the full 12 bits of higher resolution - meaning that the DSO is sampling at the full (or nearly full) speed of 1GSa/s - 2GSa/s - at least until 1ms/div.


--- Quote ---Yes there is a filtering effect, but this is firstly applied to the "oversampled" data rather than the final trace samples.
--- End quote ---

Six of one - half dozen of the other. It makes no difference whatsoever other than to the extent of how the successive sample decimation (High Res) is combined with the standard peak-to-peak decimation that is normally used between sample and display memory.


--- Quote from: Galaxyrise on November 17, 2013, 07:50:32 pm ---Naturally, now that I know more what it does, it can be a useful tool.  One just has to be aware that at 1ms/div + high res, the effective bandwidth (on screen) is like 5kHz.

--- End quote ---

That's the general idea, but you're off by a factor of about x4. The effective 3dB bandwidth (when using 14MB/AUTO + High Res) is roughly something like:
1ms = 20kHz
500us = 40kHz
200us = 100kHz
100us = 200kHz
50us = 400kHz
20us = 1MHz
10us = 2MHz
5us = 4MHz (the approx. maximum of any 2GSa/s DSO doing successive sample decimation to 12 bits)
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